PROPERTY, PLANT AND EQUIPMENT - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 7,649,791	$ 8,486,200
Accumulated depreciation	(4,275,556)	(4,839,859)
Property, plant and equipment, net	3,374,235	3,646,341
Assets held-for-sale	13,076	0
Drilling services equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 6,479,582	7,313,234
Drilling services equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	4 years
Drilling services equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	15 years
Tubulars
Property, Plant and Equipment [Line Items]
Estimated useful lives	4 years
Property, plant and equipment, gross	$ 606,036	615,281
Real estate properties
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 43,398	43,389
Real estate properties | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	10 years
Real estate properties | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	45 years
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 468,392	464,704
Other | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	2 years
Other | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	23 years
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 52,383	$ 49,592